In order to produce cast strip products in the form of sheet, conventionally continuous casting installations are used. A typical continuous casting installation is shown in FIGS. 1 and 2 in which reference numerals 1a and 1b denote a pair of or front and back casting rolls arranged horizontally and side by side and rotatable downward and toward to each other, the casting rolls 1a and 1b being adapted to be internally cooled through communication of cooling fluid in the rolls. Thus, casting rolls 1a and 1b provides a continuous casting machine 1. In conventional continuous casting, a typical cast strip thickness is 30 mm or more; however, in recent roll casting, a cast strip thickness may be thinner and may be 15 mm or less.
Reference numeral 2 denotes a molten metal nozzle arranged above a molten metal pool between the casting rolls 1a and 1b; 3, a tundish arranged above the nozzle 2 to feed molten metal 4 to the nozzle 2; 5, side weirs arranged laterally and oppositely of the casting roll 1a and 1b to abut on ends of the casting rolls 1a and 1b so as to prevent the molten metal 4 from leaking from the molten metal pool; 6, a cast piece or strip in the form of thin sheet and formed by cooling of the casting rolls 1a and 1b; 7, pinch rolls arranged downward of the casting rolls 1a and 1b to draw out the strip 6; and 2a, side flow channels formed on opposite sides of the molten metal nozzle 2.
In the above-mentioned continuous casting machine 1, molten metal 4 is fed from the molten metal nozzle 2 to between the casting rolls 1a and 1b to form the molten metal pool, the molten metal 4 being cooled by the casting rolls 1a and 1b and being delivered as the strip 6 from between the rolls through rotation of the latter.
However, when a continuous casting operation is effected by the above-mentioned continuous casting machine 1, in triple point 8 provided by the rotated casting rolls 1a and 1b, side weirs 5 and molten metal 4 as shown in FIG. 3, a solidified shell 9 integrally develops on peripheries of the casting rolls 1a and 1b and on inner surfaces of the side weirs 5. Rotation of the casting rolls 1a and 1b may cause such solidified shell 9 to be plucked away to produce triple point problems such as formation of infinitely-lacking shape defects on widthwise edges of the strip 6, flow out of the unsolidified inner molten metal 4 and fractures of the strip 6.
To overcome this, recently, formation of the solidified shell 9 on the side weirs 5 has been prevented such that part of the molten metal 4 fed from the nozzle 2 to the molten metal pool is made to flow via side flow channels 2a positively to the triple point 8 regions to thereby prevent formation of the solidified shell 9 on the side weirs 5. In this respect, the fed amount of the molten metal 4 is controlled depending upon thickness and production speed of the strip 6 to be cast so as to retain a pool surface height H constant.
However, in the above-mentioned conventional system, too much flow rate of the molten metal 4 fed to the triple point 8 regions may cause the solidified shell 9 on the casting rolls 1a and 1b to be also melted, resulting in shape defects 11 such as droplet-like leaks and bulges on the widthwise edges of the strip 6; too little flow amount to the triple point 8 may cause the above-mentioned triple point problems.
Any try to control the flow rate of the molten metal 4 fed to the triple point 8 would vary the pool surface height H, resulting in deviation in supply position of the molten metal 4 directed to the triple point 8 for prevention of the triple point problems to thereby produce the above-mentioned shape defects 10 and 11.
Therefore, conventionally, control is made to retain the pool surface height H constant; the fed amount of the molten metal 4 to the triple point 8 is not controlled at all. As a result, any change of the above-mentioned casting conditions may produce shape defects 10 and 11 on the widthwise edges of the strip 6, leading to deterioration of product quality, difficulties in succeeding operations such as rolling and resultant increase in cost. Especially, upon startup of a casting operation, the molten metal 4 may be solidified in a flow channel in the molten metal nozzle 2 to narrow the section of the flow channel and reduce the flow rate so that the triple point problems occur significantly, resulting in problems such as reduction of yield of the strip 6.
Continuous casting machines for solving such problems have been proposed as shown in JP-63-317240A. In such continuous casting machine, as shown in FIG. 4, a continuous casting machine 1 comprising two casting rolls 1a and 1b defines together with opposite side weirs 5 a molten metal pool; and a tundish 3 arranged above the pool is formed with a main flow channel 3a and side flow channels 3b which feed the molten metal 4 to the opposite triple point regions of the molten metal pool, the flow rates of the molten metal 4 flowing through the respective flow channels 3a and 3b being individually controlled by control members 14 and 15 vertically movable through actuators 12 and 13, respectively.
In the case of shape defects 10 being generated on the widthwise edges of the strip 6 in the continuous casting machine 1 of FIG. 4, opening degrees of the side flow channels 3b are controlled by the control members 15 to control the fed amount of molten metal to the triple point regions so as to eliminate the shape defects 10 on the widthwise edges of the strip 6. Any variation of the pool surface height H due to variation in the fed amount of the molten metal to the triple point 8 is absorbed by controlling the opening degree of the main flow channel 3a through the control member 14 to control the amount of the molten metal flowing through the mail flow channel 3a, thereby maintaining the pool surface height H constant.
The side flow channels 3b of the tundish 3 shown in FIG. 4 are generally narrow and unstable and may be clogged when the molten metal 4 flow through them; as a result, they have insufficient effect on compensation of the shape defects (flaws) 10 on the widthwise edges of the strip 6. Therefore, in the case of the strip 6 being rolled by a downstream rolling mill, this may cause frequent meanderings and/or fractures of a strip product produced by rolling of the strip 6. Such shape defect problems are especially critical in the case of the cast strip thickness of 15 mm or less since meanderings further tend to occur upon rolling due to the thin cast strip thickness, resulting in increase in number of troubles.
When the casting rolls 1a and 1b are deformed into convex as shown in FIG. 5 under the influence of for example heat, the cast strip 6 may have sectional shape as shown in FIG. 6 with convex portions 6a at widthwise edges due to edge-up; when the casting rolls 1a and 1b become concave as shown in FIG. 7 due to grinding, the strip 6 may have sectional shape as shown in FIG. 8 with concave portions 6b at widthwise edges due to edge-down.
As a result, in the case of the strip 6 being rolled by the downstream rolling mill, elongation ratio of the strip may be nonuniform widthwise, resulting in generation of shape defects. Such edge-up or -drop may be generated frequently dissymmetry widthwise. Furthermore, structurally with respect to plastic mass flow of the strip product rolled, elongation longitudinally of the strip may increase in comparison with that widthwise of the strip, resulting in increased flatness defectiveness of the strip longitudinally of the strip.
In view of the above, the invention has its object to prevent any troubles in rolling of a strip or prevent any flatness defectiveness of the strip after rolling even if the strip produced by a continuous casting machine may have thickness defects on widthwise edges thereof due to flaws and/or edge-up or -drops.